Bring on the beach days and lazy picnics: Summer has arrived. But be careful. Too much of a good thing can be deadly.

More than 300 people die each year from heat-related diseases — and thousands get very sick, according to the Centers for Disease Control and Prevention.

The three most common heat problems can affect anyone at any age: heat rash, heat exhaustion and heat stroke — but they're 100 percent preventable.

Before you step into the sun, understand the dangers caused by heat.

Heat rash

This skin irritation is caused by excessive sweating during humid heat, said Anne Chapas, assistant clinical professor of dermatology at NYU, and dermatologist at the Laser and Skin Surgery Center of New York. Heat rash occurs when the sweat glands are blocked, which causes sweat to accumulate under the skin.

It looks like a cluster of small blisters or pimples — and tends to occur in spots where you have more hair follicles and pores — so under the arm pits, in elbow creases, on the chest, neck or shoulders.

"Most people do not have severe symptoms but are more bothered by the appearance," Chapas said.

You can treat heat rash by keeping the area dry and using over-the-counter products such as baby powder and anti-fungal sprays or creams (look for anything used to treat athlete's foot or jock itch) to lessen the discomfort. If the blisters appear extreme or don't go away after a few days, you can contact your dermatologist, who may prescribe a topical antibacterial or oral antibiotic, said Joseph Fowler Jr., a dermatologist and clinical professor of dermatology at the University of Louisville.

To avoid heat rash when you're in the sun, wear breathable fabrics such as cotton or linen, and avoid using greasy gels or lotions which may block sweat glands.

Heat exhaustion

A long day at an outdoor festival can do this to you.

Exposure to heat plus dehydration can lead to this nasty condition. Flu-like symptoms plus a throbbing headache, cool skin, chills and a pale, weak pulse can all occur, said Dr. Bob McNamara, chairman of the Department of Emergency Medicine at Temple University School of Medicine.

Treat it by drinking water and finding a cool area to sit.

Continue to stay out in the sun, and the relatively harmless heat exhaustion can turn into the deadly heat stroke.

Heat stroke

This is the mother of all heat-related diseases, and can cause death or serious disabilities.

It happens quickly when you ignore the symptoms of heat exhaustion, and your body is unable to cool down.

Quite simply, "It's a medical emergency," said Lawrence Armstrong, professor of exercise and environmental physiology at the University of Connecticut, and author of "Exertional Heat Illnesses."

Normally, your body floats around 98.6 degrees. But it rises to upwards of 103 if you're out in the sun — and especially if you're exercising in the heat. Your body simply can't dissipate all of its heat into the environment, so it starts storing the heat. All that warmth in your body is too much for your organs, which will start malfunctioning, and your body will lose its ability to sweat.

Many people will start hallucinating, getting dizzy and even combative, said Rahul Khare, an emergency room physician at Northwestern Memorial Hospital.

The biggest problem with heat stroke is that most people who have it have no idea they're in any danger, Khare said.

There are some immediate things you can do to get someone out of danger, however. First, call 911 and get them out of the heat. Next, grab a drink. Khare recommended a sports drink like Gatorade, because it would have electrolytes — sodium and potassium — that the body loses through sweating. If you don't have a sports drink handy, water will do.

Then, get them under cool water, and place ice-packs on their arm pits, groin, head and neck, Armstrong said.